This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP00/02401 which has an International filing date of Apr. 13, 2000, which designated the United States of America.
The present invention relates to a method for producing rubber compositions. More particularly, it relates to a method which makes it possible to produce rubber compositions having high productivity and quality stability and excellent in appearance, fluidity and mechanical strength.
There have already been known thermoplastic elastomer compositions (rubber compositions) obtained by so-called dynamic crosslinking, namely, crosslinking a radical crosslinkable elastomer and a resin having no radical crosslinkability, such as polypropylene, under melt-kneading them in an extruder in the presence of a radical initiator, and these compositions are widely employed for uses such as automobile parts.
As these rubber compositions, there have been known ethylene-propylene-diene rubbers (EPDM) or olefin elastomers produced using metallocene catalysts (JP-A-9-104787). However, such compositions are not necessarily sufficient in mechanical strength, and by increasing the amount of elastomer component, the mechanical strength can be improved, but appearance and fluidity are considerably deteriorated.
On the other hand, as conventional techniques to improve properties of dynamically crosslinked olefin rubbers, there has been disclosed a method for producing thermoplastic rubber compositions by crosslinking an olefin rubber and an olefin resin, characterized in that a dynamic surface treatment is carried out repeatedly a plurality of times (JP-A-58-145741). However, this patent publication does not mention at all that the properties are remarkably improved by dividing a crosslinking agent or a crosslinking agent and a rubber into parts and adding the parts separately at a specific crosslinking degree.
Therefore, it is demanded to provide practically acceptable rubber compositions excellent in appearance, fluidity and mechanical strength.
In view of the problems mentioned above, the object of the present invention is to provide a method for producing rubber compositions having high productivity and quality stability and excellent in appearance, fluidity and mechanical strength.
As a result of intensive research conducted by the inventors on improvement of rubber compositions, it has been found that appearance, fluidity and mechanical strength are surprisingly markedly improved by dividing a crosslinking agent into parts and adding them separately, thereby to carry out sequential crosslinking. Thus, the present invention has been accomplished.
That is, the present invention relates to a method for producing a rubber composition crosslinked to a crosslinking degree of 50% or higher by mixing a crosslinkable rubber (A) and a thermoplastic resin (B) and carrying out crosslinking with a crosslinking agent (C), where the crosslinking agent (C) is divided into parts and they are added separately to carry out sequential crosslinking in which the crosslinking is carried out to a crosslinking degree of 5-45% by the first addition of the crosslinking agent (C) and, furthermore, the crosslinking is carried out to a crosslinking degree of 50% or higher by the further addition of the crosslinking agent (C), and, moreover, the present invention relates to the above-mentioned method where the crosslinkable rubber (A) is also divided into parts and they are separately added as well as the crosslinking agent (C).